The neurosensory components of the ear that extract vestibular and auditory information and conduct it to the brain are hair cells and sensory neurons. Our short term goal is to analyze in vivo the molecular biology of neurosensory cell formation and survival that generates the most crucial cell types of the ear and connects them. Our long term goal is to use this information for neurosensory regeneration and to guide growing afferents to reach either regenerated hair cells or stimulating electrodes of cochlear implants. We will focus in this application on three steps required to develop and connect hair cells and sensory neurons. These are neurosensory fate determination, neurosensory fate acquisition and neurosensory connection formation. All of these events are influenced by a cascade of genes expressed in the growing otocyst. We will concentrate on the role of three genes and their possible interactions that determine the neurosensory region of the ear using composite mutant mice that are null or heterozygous for various combinations of three genes (Aim1). We will continue our investigation on the role of the three major bHLH genes in various combinations that are null or heterozygous or in newly generated transgenic line to dissect the molecular mechanisms of hair cell and sensory neuron fate acquisition (Aim2). Aim3 will extend our investigations into neonates and postnatal mice on the role of neurotrophins for neuronal fiber guidance through the use of transgenic misexpression and targeted elimination of neurotrophins in the ear. AIM 1 will specify the role of FgflO, Gata3 and Foxgl in neurosensory development of the ear. These three factors are known to interact in neurosensory development in the central and peripheral nervous systems of mice and flies, but their interactions in the ear are not clarified. AIM 2 will test the molecular interactions of bHLH genes (Atohl, Neurogl, Neurodl) in hair cell and sensory neuron development. Identifying multipotent precursors, that can be manipulated to generate all neurosensory cells could greatly benefit current attempts of restoring neurosensory hearing loss. AIM 3 will molecularly dissect the role of neurotrophins in neonatal and postnatal fiber reorganization using conditional Bdnf ear mutations in combination with transgenic misexpressors. Such information is crucial to govern the use of neurotrophins for regeneration, plasticity and maintenance of adult innervation.